In general, this invention relates to systems for controlling automatic transmissions. More particularly, it relates to an accessory for controlling the effect of intake manifold pressure on a transmission vacuum modulator control unit.
It is a conventional practice to employ a vacuum modulator control unit (vacuum modulator) to control the shifting of an automatic transmission. The vacuum modulator has a port, referred to herein as a load port, for receiving a pressure signal. Within the vacuum modulator there is a diaphragm against which the pressure signal acts and a valving arrangement responsive to the diaphragm for controlling line pressure in a hydraulic circuit involved in controlling the shifting of the transmission.
Conventionally, the load port is coupled by a vacuum line to a port, referred to herein as a source port, in the intake manifold of the engine.
In a naturally aspirated engine, the intake manifold pressure is, during idle or during deceleration conditions, quite low. Expressed equivalently, intake manifold vacuum is quite high during such operating conditions. When the throttle is opened to cause the engine to develop more power, the intake manifold pressure increases to a level slightly below the ambient atmospheric pressure.
Vacuum modulators, although differing in design detail from one manufacturer to another, are commonly designed so that the vacuum modulator is compatible with variations of intake manifold pressure within the foregoing range in fulfilling its role in controlling the proper shifting of the transmission.
In contrast to a naturally aspirated engine, an engine equipped with a supercharging device such as a turbocharger has temporary periods of operation during which intake manifold pressure exceeds ambient atmospheric pressure. Whereas boosting intake manifold pressure has proven advantageous for the purpose of maximizing engine output power, it has produced adverse side effects on the vacuum modulator operation. These adverse side effects include the occurrence of undesired downshifts at high engine and vehicle speeds; of delays in upshifts, especially at wide open throttle; harsh shifting; undesirably high operating pressure in the internal hydraulic circuit; internal damage to the transmission and overreving of the engine causing engine damage and turbocharger failure; and possible damage to the vacuum modulator.
There have been proposed a number of approaches directed to solving this problem, but none have proved satisfactory. According to one of these unsatisfactory approaches, there is provided, among other things, a check valve that is connected by vacuum lines between the source port and the load port. The check valve by itself protects the vacuum modulator against damage that might otherwise occur from having a positive (i.e., higher than atmospheric) pressure applied to its load port. However, introducing such a check valve creates other problems. One such other problem relates to the need to bleed the vacuum line that connects the check valve to the load port. Unless such bleeding is provided for, the check valve will in normal operating conditions, close at the point of highest vacuum and remain closed thereby trapping high vacuum in the vacuum modulator while intake manifold pressure varies within its non-supercharged range, thereby undesirably precluding the vacuum modulator from responding to intake manifold pressure. In view of this, there has been provided in accordance with this prior art approach a separate vacuum line. This separate vacuum line is connected at one of its ends to a tap or T connection located between the check valve and the vacuum modulator, and at its opposite end to the inlet side of the compressor within the turbocharger. In this arrangement, the operation of the vacuum modulator is undesirably influenced by the pressure at the turbo inlet side of the compressor, with the adverse result that the shift points of the transmission are inaccurate. Separately, there are adverse effects on the engine.
According to another unsatisfactory approach, there is provided a bypass assembly having a bore therethrough to provide a continuous communication path for use in coupling the source port to the load port. The accessory has an exhaust port communicating with the continuous communication path, and a flexible flap which normally covers the exhaust port but which is forced away therefrom when the pressure in the bore exceeds atmospheric pressure. This causes the highly combustible vapor from the intake manifold to be vented into the engine compartment and accordingly creates a hazardous condition.